In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there have been, and continue to be, efforts toward scaling down device dimensions (e.g., at sub-micron levels) on semiconductor substrates. In order to accomplish such high device packing densities, smaller and smaller feature sizes are required. This includes the width and spacing of interconnecting lines, spacing and diameter of contact holes, and surface geometry such as corners and edges of various features. With an ever increasing number of integrated circuit features being formed on a semiconductor substrate, the amount of reticle or photomask contamination that may be tolerated without inducing defects in resulting semiconductor substrate has decreased. The importance of detecting contamination on a reticle or photomask, prior to semiconductor substrate exposure, has increased correspondingly.
Close spacing between adjacent small features requires high-resolution optical lithography. Optical lithography refers generally to the technology which enables etching patterns on a substrate through use of photographic development of images that have been attached onto the surface of the substrate using a mask. This technique is commonly used for integrated circuit fabrication in which a silicon structure is uniformly coated with a light-sensitive film, and an exposing source such as optical light, x-rays, or an electron beam illuminates selected areas of the surface through an intervening master template. The intervening master template is generally a reticle or photomask for a particular pattern. The lithographic coating is generally a light-sensitive coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern. Exposure of the coating through a reticle or photomask causes the image area to become either more or less soluble, depending on the coating, in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
The process of manufacturing semiconductors employing reticles typically consists of more than one hundred steps. Generally, the process involves creating several patterned layers on and into a substrate that ultimately forms the complete integrated circuit. The patterned layers are created, in part, by the light that passes through the reticles. In order to achieve desired yield rates, reticles should be maintained relatively free of contamination throughout their use in the imaging process.
Unfortunately, during the manufacturing process, reticles may become contaminated. A contaminated reticle that goes undetected can result in the production of defective semiconductor substrates thereby reducing yield and efficiency of the fabrication process. With an ever-increasing number of integrated circuit features being formed on a semiconductor substrate, the importance of maintaining a reticle relatively free from contamination is increasing important. As the device feature dimensions reduce, lesser amounts or degrees of contamination may result in the production of defects in the semiconductor substrate. Therefore, detection of contamination is increasingly important to production line efficiency and yield.
Further exacerbating the situation, the trend towards higher device densities and smaller feature sizes requires substrate exposure at shorter wavelengths (193 nm, 157 nm). At such shorter wavelengths, defects such as haze or soft contamination grow on a reticle at a much faster rate than exposures at standard I-line (365 nm) or deep ultraviolet (248 nm) wavelengths. Soft contaminants and haze on a reticle block or reduce the amount of light that successfully passes through the clear portions of the reticle, thereby reducing the degree of exposure at the corresponding areas of the semiconductor substrate. The reduced exposure levels often result in corresponding defects in the exposed substrate.
Detection of contamination on a reticle is even more critical in step-and-repeat systems. Step-and-repeat systems repeatedly print a circuit pattern appearing on a reticle onto an area of a semiconductor substrate having photosensitive coating. This is accomplished by repeatedly projecting an image of the reticle onto successive portions of the semiconductor substrate. Such projection systems are used for device fabrication having reductions of greater than 1, typically 10:0 and 5:1, wherein the reticle contains a single copy or multiple copies of the device pattern to be employed. In step-and repeat systems, a defect on the reticle will be printed at every exposure location as the pattern is stepped and exposure is repeated across the semiconductor substrate. Thus, a semiconductor substrate defect resulting from a contaminated reticle will be created in all exposure locations on the semiconductor substrate. This significantly reduces yield and can result in an entire semiconductor substrate being unusable.
Conventional methods for contamination detection involve a man-in-the-loop inspection process whereby a person visually inspects the reticle or photomask at periodic intervals (e.g. once per day, once per week, etc.). Such methods are costly, time consuming, subject to the frailties of the human inspector, and cannot detect contamination or damage that may occur immediately following an inspection. For example, it is possible that a reticle may become contaminated or damaged while being moved from the inspection station to the lithographic imaging system. Furthermore, a reticle could be damaged while it is being loaded in the lithographic imaging system, may become contaminated while the patterning system is operating, or may become defective due to degradation of the reticle material. Such problems may remain undetected until the reticle is unloaded and reinspected, thereby allowing a significant amount of product to be processed with a resulting defect. Such problems drive production line yield and efficiency down and correspondingly increase product costs. More repeatable, consistent and timely methods of detecting defects are desirable to provide repeatable, consistent, cost effective and timely results.